What does this PR do?

Adds a clarifying section explaining when linkedSignal should be preferred over computed.

The documentation already explains how linkedSignal works, but it was not explicitly clear when it should be chosen instead of computed. This addition clarifies that linkedSignal is appropriate when the derived state must remain writable or preserve user-driven updates while still reacting to a source signal.

Why is this change needed?

Readers may understand how linkedSignal behaves, but still be unsure about when to use it over computed. This clarification helps reduce that ambiguity without restructuring or expanding the guide significantly.

Scope

• Documentation only
• No behavioral changes